Transportation's Role in Reducing U.S. Greenhouse Gas Emissions ...

The severity of congestion at a
bottleneck is related to its physical
design. Some facilities were originally
constructed many years ago using
designs that were appropriate when
they were built, but are now
considered antiquated. Others have
been built to extremely high design
specifications and are simply
overwhelmed by traffic.
The costs of reducing bottlenecks vary
considerably. In many cases,
bottleneck relief can occur through
relatively low-cost strategies such as:
hard shoulder running (using a
shoulder as a through lane during
peak or emergency periods);
restriping to increase the number of
lanes; paved shoulders; or median
barrier treatments (e.g., cables, Jersey
Transportation’s Role in Reducing U.S. Greenhouse Gas Emissions: Volume 2
barriers). Somewhat higher-cost strategies include auxiliary lanes, especially between two
closely spaced interchanges; collector-distributor roads; and added lanes. In some cases,
major reconstruction of interchanges – at high cost – is necessary. Traffic management
strategies such as ramp metering and active traffic management (lane control, speed
harmonization, and queue warning), which are considered elsewhere in this report, also
can be considered to be bottleneck relief strategies. Many major bottleneck fixes include
“packages” of improvements, which combine major reconstruction, low-cost treatments,
transit service, and improved operations. Improvements at one location are often done in
conjunction with other improvements across an entire corridor. Specific bottlenecks may
be under the jurisdiction of either State or local transportation agencies and subject to
improvement through the transportation planning and programming process.
Magnitude and Timing of GHG Reductions
Bottleneck Relief
Benefits: Low: 1-6 mmt CO2e in 2030
• Low benefits reflect induced demand effects, but
assume that all capacity improvements would be
fully financed by user fees, which would partly
mitigate these effects
• Estimates do not include GHG emissions from
construction activities or delay during construction
Direct Costs: Not available
Net Included Costs: Not available
Confidence in Estimates: Low
• Significant uncertainty over induced demand effects
Key Co-Benefits and Impacts: Positive
• Significant time savings to travelers
Feasibility: Moderate - High
• Established transportation project development
process. Some projects may encounter
environmental constraints
Key Policy Options:
• Funding
Bottlenecks are the source of recurring congestion. Though occurring at specific locations,
they can influence many miles of highways as queuing occurs because of the traffic flow
breakdown at the bottleneck points. Therefore, bottleneck relief reduces congestion
delays and changes drive cycles (both acceleration/deceleration profiles as well as
average speeds), potentially improving fuel efficiency and reducing GHG emissions. 21
However, bottleneck relief strategies also result in some amount of induced travel. This
21 Impacts are specific to each site. Based on work done for FHWA using microscopic traffic
simulation, for every hour of vehicle delay reduced, on average 0.62 gallons of fuel are saved by
autos and 1.94 gallons are saved by large trucks (SAIC, 1993).
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